When it comes to weight training, there is no one size that fits all. The best weight for your dumbbells depends primarily on your fitness goals and your previous experience with weight training. According to the National Strength and Conditioning Association, you need to lift weights heavy enough to fatigue your muscles within a specific repetition range. So, if you do not feel "the burn" when you have completed your sets, you are not lifting a heavy enough dumbbell.
The American Council on Exercise recommends that beginners start with a weight they can lift about 12 to 15 times for one to two sets. This is generally two to 15 pounds, depending on the muscle group. This helps develop baseline musculature and strength, plus proper technique and rhythm. Exercise with this repetition and weight range for about four weeks, then gradually progress according to your goals.
The weight of a dumbbell you select for an exercise varies according to the muscle groups. When doing bicep curls, you can use 5 to 8 pound weights, while the exercises for the weaker triceps muscles, such as the triceps kickbacks, are best performed with 2 to 5 pound weights. Meanwhile, squats would be performed with weights up to 45 pounds, as your legs and glutes are significantly stronger than your arm muscles.
Generally, those who want to train for muscular endurance are distance athletes, such as marathon runners or triathletes, or people who need muscular endurance in their work. To train for endurance, you need a dumbbell size that will fatigue your muscles in about 15 to 20 repetitions. Begin with a light weight, between 2 and 5 pounds. If you can easily do 20 repetitions, increase the dumbbell weight. Your goal is to find the maximum weight that still allows you to do 20 repetitions, but maxes out your endurance. This type of training doesn’t specifically work on increasing muscle mass -- although that may be a side effect. -- but more on increasing the amount of work your muscles are capable of over a long period.
"Muscle hypertrophy" means building muscle size. The best repetition range for building muscle mass is three sets of eight to 12 repetitions two to three time a week, using slightly more weight than that in the beginner phase. Depending on your workout status and what muscle groups you are exercising, dumbbells for hypertrophy should be between 10 and 20 pounds.
Building strength is done at a much higher intensity and weight than the hypertrophy stage. If strength is your goal, increase sets to about three or four. Increase your dumbbell weight so you are now maxing out between six and 10 repetitions. The weight of the dumbbell varies according to the exercise. If you're performing shoulder presses, begin with 2 to 5 pound weights. If you're doing squats, begin with no weights, then increase the weight up to 45 pounds, depending on your fitness level. Since these are higher intensity exercises, get adequate rest between sets, ideally about one to two minutes to let your muscles recover.
Developing power is the goal for football players, Olympic lifters, wrestlers and other athletes who require bursts of strength over very short periods. This is the most intense form of weight training, and the dumbbells are very heavy. Begin with 10 to 20 pound dumbbells and work your way up to the heavier weights to avoid injury to your wrists, arms and shoulders. Do three to six sets of about three to six repetitions. Due to the high intensity of these exercises, give yourself two to three minutes of rest between each set.